The terahertz frequency spectral range is a relatively underdeveloped band of the electromagnetic spectrum. The terahertz band is bordered by the infrared on the short-wavelength side and millimeter-waves on the long-wave length side. The terahertz band encompasses radiation having a frequency range of 0.3 to 10 THz and wavelengths between about 30 micrometers (μm) and 1 millimeter (mm). The terahertz band is sometimes referred to by practitioners of the art as the far infrared (FIR).
Many materials that are opaque to wavelengths shorter then 30 micrometers are either transparent or semi-transparent in the terahertz band. Such materials include plastic, textiles, paper, cardboard, wood, ceramics, opaque glasses, semiconductors, and the like. Radiation at longer wavelengths, for example, millimeter waves have better transmissivity than terahertz radiation in these materials but the longer wavelengths are unsuitable for use in high resolution imaging systems. Further, such materials do not have much spectral content, i.e., characteristic absorption lines, in these longer wavelength regions that would allow one to be easily distinguished from another.
Terahertz radiation is not an ionizing radiation, so it does not have the potential to present health problems as would, for example, X-radiation (X-Rays). Terahertz radiation can be propagated for much longer distances in the atmosphere than X-rays, for example, several meters, and does not cause damage to electronic devices and unexposed film. In addition to offering a higher potential resolution in imaging than millimeter waves, terahertz radiation also offers a potential to provide sharper differentiation between different materials superimposed on one another and, accordingly provide higher contrast images than would be possible with millimeter waves.
It would be advantageous to exploit the imaging potential of terahertz radiation in security apparatus for examining persons, luggage or packages for concealed objects or substances. Such objects and substances could include explosives, guns, knifes, drugs, biological agents, and the like. Theses objects or substances could be concealed under a person's clothing, or in non-metallic containers or luggage to be transported in airplanes, trains, buses, ships.